Certain malleable pure elemental metals, such as cobalt, nickel or copper, are also highly malleable when formed into pure metal powders. Attempts to reduce the average particle size of such malleable powders are often futile, as the powder particles are easily deformed and tend to flatten when subjected to particle size reduction processes.
U.S. Pat. No. 2,721,135 to Wimberly discloses a method of producing metal powders which have a closely controlled particle size distribution for use in powder metallurgy applications (column 1, lines 31-35). Too great a fraction of fine-size particles may make them unsuitable for such powder metallurgy applications (column 1, lines 53-65). An objective of the Wimberly process is thus to increase the average particle size of the metal powders as well as to closely control the particle size distribution.
The disclosed source of these powders is a metal-bearing material, typically a metallic oxide from an ore such as magnetite (column 2, lines 68-71). In the Wimberly process, the metallic oxide is sintered, an effect of which is to increase the particle size of the oxide powder (column 2, lines 36-40). The sintered material is then crushed and/or ground to the desired particle size distribution (column 2, lines 17-20 and lines 47-50). The ground oxide is then reduced by means of a reducing agent (column 2, lines 50-52), an effect of which is to agglomerate the ground oxide particles (column 2, line 53). The reduced material is then reground or milled to obtain a metal powder having the desired particle size characteristics (column 2, lines 53-56). The only metal disclosed in Wimberly is iron.
Methods for producing fine cobalt metal powders are also disclosed in U.S. Pat. Nos. 4,798,623 to Cheresnowsky, 4,705,559 to Miller, 4,348,224 to Gingerich et al., and 4,093,450 to Doyle et al. In these patents, a cobalt-containing precipitate is obtained from aqueous solutions of cobalt-containing materials. The precipitate is then reduced to form cobalt metal powders.